That should be fine, but I would suggest creating regular backups.
Notes:
Yes, that should be true (though I've only tested import with small JEX archives). It should also be possible to import the JEX archives from the Joplin desktop app.
Note: I would suggest using Chrome or Safari instead of Firefox (database access in Firefox is very slow).
From "Where is my data stored?" in the web app FAQ:
Like Joplin Mobile, Joplin Web is local-first. Notes and attachments are stored locally on your computer, but can optionally be synced with one of the supported sync targets. As a result, Joplin Web can be used offline.
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